This invention relates to a process for extracting sulfur from a gas containing hydrogen sulfide and sulfur oxides. The invention is particularly adapted for desulfurizing exhaust gas from a Claus unit. The process also works on other sulfur-containing gas streams, e.g., light, saturated hydrocarbons, hydrogen or carbon monoxide gas streams containing H2S and/or sulfur oxides.
In the Claus process, elemental sulfur is produced by reacting H2S and SO2 in the presence of a catalyst. The Claus system uses a combustion chamber which, at 950xc2x0 to 1,350xc2x0 C., converts 50 to 70% of sulfur contained in the feed gas into elemental sulfur. Sulfur is condensed by cooling the reaction gas to a temperature below the dew point of sulfur, after which the remaining gas is heated and further reacted over a catalyst. Normally, the gas passes through at least two such Claus catalyst stages.
The different stages of the process may be represented by the following equations:
H2S+3/2O2xe2x86x92SO2+H2Oxe2x80x83xe2x80x83(I)
2H2S+SO2xe2x86x923Sn+2H2Oxe2x80x83xe2x80x83(II)
The overall reaction is:
3H2S+3O2xe2x86x923Sn+3H2Oxe2x80x83xe2x80x83(III)
Below 500xc2x0 C., the symbol n has a value of approximately 8.
The final Claus exhaust gas still contains small amounts of H2S, SO2, CS2, carbon oxysulfide, CO, and elemental sulfur in the form of a vapor or mist. The exhaust gas can be subjected to post-combustion to convert substantially all sulfur species to sulfur oxides (SO2 and SO3), which are then emitted into the atmosphere.
Sulfur emitted as sulfur oxides (xe2x80x9cSOxxe2x80x9d) into the atmosphere with the exhaust gas may amount to 2-6% of the sulfur contained in the feed gas in the form of H2S. In view of air pollution and the loss of sulfur involved, further purification is imperative.
Claus after-treatments have been developed. These are carried out after the last Claus stage or after the post-combustion. These after-treatments are, however, complicated and expensive or inadequate.
One after-treatment, carried out before post-combustion, seeks to achieve by catalytic conversion as complete a reaction as possible between H2S and SO2. The reaction temperature is lowered to below the condensation point of sulfur, whereby the reaction equilibrium corresponding to equation II is shifted to form sulfur. A distinction is made between dry processes using alternating reactors in which the catalyst is intermittently charged with sulfur and discharged, and processes where H2S and SO2 react in a high-boiling catalyst-containing liquid to form elemental sulfur which is drawn off continuously as a liquid product.
Unfortunately, in these processes any deviation from the optimum H2S:SO2 ratio in the Claus exhaust gas results in a reduced sulfur yield. No appreciable conversion of sulfur compounds such as COS and CS2 occurs. Sulfur recovery efficiency of Claus using this form of after-treatment is limited to 98-99%. Cyclic operation, with alternating reactors, requires at least two reactors and much valves and piping.
A second after-treatment catalytically hydrogenates SO2 and S with H2 and CO while COS and CS2 are simultaneously hydrolyzed with H2O into H2S which can be treated conventionally.
Hydrogenation/hydrolysis does not require a stoichiometric H2S/SO2 ratio in the Claus exhaust gas. It almost completely converts COS and CS2 so that sulfur yields of more than 99.8% can eventually be obtained. This process incurs high capital expenditures for elaborate apparatus. It also consumes substantial energy. Recycle of H2S reduces the Claus system capacity, while the production of waste water containing harmful constituents presents additional problems. In addition, the treatment (such as amine absorption) used to remove H2S is generally ineffective for removing unconverted COS and CS2. Total emissions of reduced sulfur species are typically around 10 ppm by volume with this after treatment.
A third after-treatment oxidizes all sulfur compounds into SOx which is then further processed. Thee processes are downstream of the post-combustion and therefore independent of the mode in which the Claus system is run. There are also dry processes, where SO2 is adsorbed and returned to the Claus unit or processed to form sulfuric acid, and wet processes, where SO2 is removed by absorptive scrubbing and further processed. For complete oxidation of COS and CS2 into SO2, the energy requirements are high and following the after-combustion, very large exhaust gas flows have to be treated.
The equilibrium conversion of the Claus reaction (equation II) may be improved by condensing out part of the water in the gas. The gas is then reheated and charged to another Claus stage to form elemental sulfur. This produces waste water which is highly corrosive due to the formation of thiosulfuric acid, polythionic acids and sulfurous acid. Processing of such waste water is expensive. Unavoidable formation of deposits of elemental sulfur also occurs during H2O condensation. Moreover, there is no conversion of COS and CS2 so the maximum recovery of sulfur is about 98%. As a result of these disadvantages, this process has not been used on a commercial scale.
Where the after-treatment involves conversion of all sulfur compounds into hydrogen sulfide, it is also known to oxidize part of said hydrogen sulfide with air into SO2 or to convert part of the sulfur produced into sulfur dioxide and thereafter catalytically to convert the remaining hydrogen sulfide with sulfur dioxide at 125xc2x0-150xc2x0 C. in fixed-bed reactors into sulfur. The sulfur loaded catalyst is regenerated by passing hot oxygen-free gases containing hydrogen sulfide through the catalyst. This avoids the disadvantages associated with the first type of after-treatment, such as dependence on H2S/SO2 ratio and COS/CS2 content in the Claus exhaust gas. Disadvantages of this process are the high capital cost and the higher H2S+SO2 input concentration for the low-temperature reactor caused by the admixture of a separately produced flow of SO2. The maximum conversion overall efficiency obtainable with this process approaches 99%.
An after-treatment process which oxidizes all sulfur compounds into SO2 is exemplified by Groenendaal et al. in U.S. Pat. No. 3,764,665 which issued on Oct. 9, 1973. This patent disclosed a process for removing sulfur oxides from gas mixtures with a solid acceptor for sulfur oxides wherein the solid acceptor is regenerated with a steam-diluted reducing gas and the regeneration off-gas is fed to a Claus sulfur recovery process. The improvement comprises cooling the regeneration off-gas to condense the water vapor contained therein, contacting the cooled off-gas with a sulfur dioxide-selective liquid absorbent, passing the fat liquid absorbent to a buffer zone and then to a stripping zone wherein the absorbed SO2 is recovered from the liquid absorbent and is supplied to the sulfur recovery process. By operating in this manner, fluctuations in the sulfur dioxide concentration of the regeneration off-gas were leveled-out and a relatively concentrated sulfur dioxide stream was supplied to the sulfur recovery process at a substantially constant rate.
Although this process supplies relatively concentrated sulfur dioxide to the sulfur recovery process at a substantially constant rate, the off-gas must be cooled and the fat liquid absorbent must be transferred to a buffer zone before the absorbed SO2 can be stripped. Therefore, what is needed is a simpler process whereby these steps are eliminated and energy costs reduced.
This invention is directed to a process for removing substantially low concentrations of sulfur from a gas stream. Via this process a solid absorbent is used to remove sulfur oxides when said absorbent is regenerated with a hydrocarbon or hydrogen reducing gas which produces an off-gas. This off-gas is passed to a Claus sulfur recovery process. Initially, an oxygen containing gas is introduced along with an exhaust or tail-gas into an incinerator under conditions sufficient to convert substantially all of the sulfur therein into sulfur oxides. Afterwards, the resultant gas with sulfur oxides therein is directed into an absorber where substantially all of the sulfur oxides are absorbed on a solid absorbent. The resultant gas is allowed to remain in the absorber for a time sufficient for a desired amount of sulfur oxides to be absorbed on the solid absorbent. Gases emitted from the absorber, which are substantially depleted of sulfur oxides, are released to the atmosphere. Gases released into the atmosphere contain less than about 2 ppm of sulfur oxides therein.
When sufficient sulfur oxides have been absorbed on the solid absorbent, absorption is ceased. Next, the solid absorbent is regenerated by contacting it with a hydrocarbon or hydrogen reducing gas under conditions sufficient to cause absorbed sulfur oxides to release thereby forming a sulfur dioxide/hydrogen sulfide off-gas and a regenerated solid absorbent. The sulfur dioxide/hydrogen sulfide, off-gas is released in a concentration sufficient to be removed or converted by a Claus sulfur recovery process. Thereafter, the off-gas is directed to a Claus sulfur recovery process where it is converted into elemental sulfur. Besides sulfur dioxide and hydrogen sulfide the off-gas may contain water and unconverted reducing gas. It is preferable to release sulfur during regeneration primarily in the form of SO2, rather than H2S, since the recycle of SO2 to the front of the Claus plant will entail a lower air demand by the Claus plant. If H2S is fed to the Claus plant in the off-gas, then extra air would be needed, which would reduce the processing capacity of the Claus plant.
It is therefore an object of this invention to concentrate sulfur contained in an exhaust or tail-gas in amounts sufficient for removal by a Claus sulfur recovery process.
It is another object of this invention to improve the efficiency for the removal of sulfur compounds from an exhaust or tail-gas emitted from a Claus sulfur recovery process.
It is yet another object of this invention to protect the environment by lowering substantially the concentration of sulfur compounds emitted to the atmosphere.
It is a further object of this invention to provide for a dry bed absorbent process for use in reducing sulfur in an exhaust or tail-gas in combination with a Claus sulfur recovery unit to obtain substantially reduced total sulfur emissions of less than about 2 ppm.
It is a still yet further object of this invention to provide for a dry bed absorbent process for removing sulfur from an exhaust or tail-gas so as to eliminate waste disposal problems which are inherent in other processes.
It is a still even yet further object of this invention to provide for a dry bed absorbent process for the recovery of sulfur from an exhaust or tail-gas and thereby obtain carbon monoxide conversion due to the process"" substantially high operation temperatures.
It is a yet still even further object of this invention to provide for a method which will convert substantially all of the sulfur compounds in a tail-gas or exhaust gas to sulfur dioxide so as to reduce the oxygen demand upon oxidation of said stream which effectively increases the treating capacity of a Claus sulfur recovery plant.